对孤立中子星1E1207.4-5209 X-射线回旋吸收线的理论分析

近来的观测表明,在射电宁静孤立中子星1E1207．4-5209的X-射线连续谱中存在几条等间隔的吸收线,能量分别为0．7、1．4和2．1keV．根据近年发展的四极近似下的量子回旋辐射理论,澄清了目前的一些疑问,肯定了它们是电子回旋吸收线,而不是质子回旋线．并从理论上确定了该中子星自转轴的空间取向．     

On the theory of X-ray pulsar radiation

The origin of hard X-ray spectrum (continuum and cyclotron lines) of pulsars in binary systems is discussed. A model of the polar region of a neutron star consisting of a hot spot in a dense plasma atmosphere with a quasi-homogeneous magnetic field and an extended accreting column in an inhomogeneous dipolar field is investigated. In the hot spot bremsstrahlung and Thomson scattering form continuum radiation, while bremsstrahlung and cyclotron scattering produce the absorption cyclotron lines. By the observed continuum intensity one can estimate the maximum distances to pulsars. Cyclotron scattering in gyro-resonant layers localized in the accreting column leads to a general attenuation of the radiation of a hot spot, but is unable to ensure the formation of cyclotron lines. For strong accretion the hot spot radiation becomes insignificant, the lines disappear and the pulsating component of an X-ray pulsar is produced by the accreting column bremsstrahlung transformed by Thomson scattering.     

The magnificent seven: magnetic fields and surface temperature distributions

Presently seven nearby radio-quiet isolated neutron stars discovered in ROSAT data and characterized by thermal X-ray spectra are known. They exhibit very similar properties and despite intensive searches their number remained constant since 2001 which led to their name “The Magnificent Seven”. Five of the stars exhibit pulsations in their X-ray flux with periods in the range of 3.4 s to 11.4 s. XMM-Newton observations revealed broad absorption lines in the X-ray spectra which are interpreted as cyclotron resonance absorption lines by protons or heavy ions and/or atomic transitions shifted to X-ray energies by strong magnetic fields of the order of 10¹³ G. New XMM-Newton observations indicate more complex X-ray spectra with multiple absorption lines. Pulse-phase spectroscopy of the best studied pulsars RX J0720.4-3125 and RBS 1223 reveals variations in derived emission temperature and absorption line depth with pulse phase. Moreover, RX J0720.4-3125 shows long-term spectral changes which are interpreted as due to free precession of the neutron star. Modeling of the pulse profiles of RX J0720.4-3125 and RBS 1223 provides information about the surface temperature distribution of the neutron stars indicating hot polar caps which have different temperatures, different sizes and are probably not located in antipodal positions.     

The energy spectra of gamma-ray bursts: old puzzles,recent analysis and new results

Gamma-ray burst spectroscopy has at times been thought of as the key to understanding these mysterious events, and at others as a source of disappointment and confusion. Early spectral measurements contained evidence for cyclotron-line-like absorption and emission features. These results were the main anchor for the galactic neutron star paradigm for the origin of gamma-ray bursts. Ginga provided striking confirmation with evidence in three bursts for cyclotron absorption with both first and second harmonics present. The absorption line energies were consistent with the teragauss magnetic fields expected for neutron stars. The BATSE experiment on CGRO has dealt a severe blow to this paradigm. BATSE has found no evidence for the expected galactic anisotropy and also no evidence for cyclotron lines. The exact degree of inconsistency between BATSE and earlier GRB line results remains a complicated and unresolved question. In general, burst spectra display a wide variety of both spectral and temporal behavior. In essentially all bursts where spectral measurements are possible, the spectrum evolves during the burst. This spectral complexity and variability has presented researchers with a challenging task. At the present time, only the broadest correlations and characterizations are in hand. This paper will; 1) give a brief historical overview of GRB spectroscopic measurements and 2) provide a summary of the current observational situation with emphasis on the BATSE/Ginga controversy.     

Study of magnetized accretion flow with cooling processes

We have studied shock in magnetized accretion flow/funnel flow in case of neutron star with bremsstrahlung cooling and cyclotron cooling. All accretion solutions terminate with a shock close to the neutron star surface, but at some regions of the parameter space, it also harbours a second shock away from the star surface. We have found that cyclotron cooling is necessary for correct accretion solutions which match the surface boundary conditions.     

On timing and spectral characteristics of the X-ray pulsar 4U 0115+63: Evolution of the pulsation period and the cyclotron line energy

An overview of the results of observations for the transient X-ray pulsar 4U 0115+63, amember of a binary system with a Be star, since its discovery to the present day (～40 years) based on data from more than dozen observatories and instruments is presented. An overall light curve and the history of change in the spin frequency of the neutron star over the entire history of its observations, which also includes the results of recent measurements made by the INTEGRAL observatory during the 2004, 2008, and 2011 outbursts, are provided. The source’s energy spectra have also been constructed from the INTEGRAL data obtained during the 2011 outburst for a dynamic range of its luminosities 10³⁷?7 × 10³⁷ erg s^?1. We show that apart from the fundamental harmonic of the cyclotron absorption line at energy～11 keV, its four higher harmonics at energies ?24, 35.6, 48.8, and 60.7 keV are detected in the spectrum. We have performed a detailed analysis of the source’s spectra in the 4–28 keV energy band based on all of the available RXTE archival data obtained during bright outbursts in 1995–2011. We have confirmed that modifying the source’s continuum model can lead to the disappearance of the observed anticorrelation between the energy of the fundamental harmonic of the cyclotron absorption line and the source’s luminosity. Thus, the question about the evolution of the cyclotron absorption line energy with the luminosity of the X-ray pulsar 4U 0115+63 remains open and a physically justified radiation model for X-ray pulsars is needed to answer it.     

On hard X-ray spectra of accreting neutron stars

Formation of the spectra of X-ray pulsars and gamma bursters is investigated. Interpretation of a hard X-ray spectrum of pulsars containing cyclotron lines is feasible on the basis of an isothermal model of a polar spot heated due to accretion to a neutron star. It has been ascertained that in the regions responsible for the formation of continuum radiation and lines the mode polarization is determined by a magnetized vacuum rather than by a plasma. Bearing this in mind, the influence of the magnetic field of a star on the wide wings of the cyclotron line and on its depth is discussed. The part played by the accreting column in the case of strong accretion (10¹⁹ el cm^–3) needed for long sustaining of the high level of X-rays from a neutron star-pulsar is studied. There occur the gaps in spectrum at frequencies close to the electron gyro-frequency and its harmonics due to the screening of the hot spot by the opaque gyro-resonant layers located within the accreting column. These gaps ensure the formation of cyclotron lines in absorption irrespective of the presence of such lines in the X-ray spectrum of a polar hot spot.The spectra of gamma-bursters recorded by Venus 11 and Venus 12 are interpreted in terms of a two-layer model of a polar hot spot. The estimates are given of the distance to some of the bursters, of the emission measure from a high-temperature layer responsible for continuum radiation and of the dispersion measure of a colder layer forming cyclotron lines in absorption. It is noted that the action of an accreting column leads generally to the radiation depression at frequencies below cyclotron lines. By the observed depression for one of the bursters the electron density of near-star accreting plasma during the burst has been directly estimated (4×10^–14 el cm^–3). Possible appearance of false cyclotron lines associated with cyclotron scattering in accreting column has been revealed.The problem of measuring the magnetic fields of neutron stars taking account of the gravitational redshift and the quantum recoil effect in emission and in absorption is discussed. Possibilty for a more precise measurement of the magnetic fields of those bursters whose spectrum contains both a cyclotron and an annihilation lines is noted.     

The complex X-ray spectrum of the isolated neutron star RBS1223

We present a first analysis of a deep X-ray spectrum of the isolated neutron star RBS1223 obtained with XMM-Newton. Spectral data from four new monitoring observations in 2005/2006 were combined with archival observations obtained in 2003 and 2004 to form a spin-phase averaged spectrum containing 290 000 EPIC-pn photons. This spectrum shows higher complexity than its predecessors, and can be parameterised with two Gaussian absorption lines superimposed on a blackbody. The line centers, E ₂≃2E ₁, could be regarded as supporting the cyclotron interpretation of the absorption features in a field B∼4×10¹³ G. The flux ratio of those lines does not support this interpretation. Hence, either feature might be of truly atomic origin.      

1E 1207.4–5209: Are the optical and X‐ray spectra blueshifted?

1E 1207.4–5209 is an X‐ray source located at the centre of the supernova remnant (SNR) PKS 1209–52 (G296.5+10.0) and is thought to be an isolated neutron star (INS) associated with the SNR. Its optical spectrum shows several absorption lines and the X‐ray spectrum exhibits three variable absorption features at what appears to be harmonically related wavelengths, the latter being interpreted as due to cyclotron resonance. However, there are several serious problems, uncertainties and difficulties in the above association (SNR/INS) and in the interpretation of the spectra. In view of these, we suggest an alternative explanation of the observed spectra in terms of blueshifts. This implies that the optical and X‐ray absorption spectra are due to the central object of the SNR in association with two separate absorption clouds ejected at successively increasing speeds. The clouds have their origins in jets resulting from the merger of two very massive compact stars. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical spectroscopy of the quiescent counterpart to EXO 0748−676

We present phase resolved optical spectroscopy and X-ray timing of the neutron star X-ray binary EXO 0748−676 after the source returned to quiescence in the autumn of 2008. The X-ray light curve displays eclipses consistent in orbital period, orbital phase and duration with the predictions and measurements before the return to quiescence. Hα and He  i emission lines are present in the optical spectra and show the signature of the orbit of the binary companion, placing a lower limit on the radial velocity semi-amplitude of   K ₂ > 405 km s⁻¹  . Both the flux in the continuum and the emission lines show orbital modulations, indicating that we observe the hemisphere of the binary companion that is being irradiated by the neutron star. Effects due to this irradiation preclude a direct measurement of the radial velocity semi-amplitude of the binary companion; in fact, no stellar absorption lines are seen in the spectrum. Nevertheless, our observations place a stringent lower limit on the neutron star mass of   M ₁ > 1.27 M_⊙  . For the canonical neutron star mass of   M ₁= 1.4 M_⊙  , the mass ratio is constrained to  0.075 < q < 0.105  .     

Constraining the neutron star equation of state using XMM‐Newton

We have identified three possible ways in which future XMM‐Newton observations can provide significant constraints on the equation of state of neutron stars. First, using a long observation of the neutron star X‐ray transient Cen X‐4 in quiescence one can use the RGS spectrum to constrain the interstellar extinction to the source. This removes this parameter from the X‐ray spectral fitting of the pn and MOS spectra and allows us to investigate whether the variability observed in the quiescent X‐ray spectrum of this source is due to variations in the soft thermal spectral component or variations in the power law spectral component coupled with variations in N_H. This will test whether the soft thermal spectral component can indeed be due to the hot thermal glow of the neutron star. Potentially such an observation could also reveal redshifted spectral lines from the neutron star surface. Second, XMM‐Newton observations of radius expansion type I Xray bursts might reveal redshifted absorption lines from the surface of the neutron star. Third, XMM‐Newton observations of eclipsing quiescent low‐mass X‐ray binaries provide the eclipse duration. With this the system inclination can be determined accurately. The inclination determined from the X‐ray eclipse duration in quiescence, the rotational velocity of the companion star and the semi‐amplitude of the radial velocity curve determined through optical spectroscopy, yield the neutron star mass. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Infrared bursts from liller I/MXB 1730-333

The observation of infrared bursts from the globular cluster Liller I has been reported by Kulkarniet al. (1979) and confirmed by Joneset al. (1980). The infrared bursts which resemble Type I X-ray bursts in their characteristics are plausibly attributed to a cyclotron maser instability operating at few tens of neutron star radii above the poles of a magnetized neutron star in a binary system. It is suggested that similar infrared bursts should in general be observable from Type I X-ray burst sources.     

Infrared emission from X-ray sources

The infrared emission from some X-ray sources is attributed to proton—cyclotron masering process operating near the polar regions of an accreting neutron star in a binary system.     

NuSTAR observations of the X-ray pulsar LMC X-4: A constraint on the magnetic field and tomography of the system in the fluorescent iron line

We present the results of the spectral and timing analysis of the X-ray pulsar LMC X-4 based on data from the NuSTAR observatory in the broad X-ray energy range 3–79 keV. Along with a detailed analysis of the source’s averaged spectrum, high-precision spectra corresponding to different phases of the neutron star spin cycle have been obtained for the first time. The Comptonization model is shown to describe best the source’s spectrum, and the evolution of its parameters as a function of the pulse phase has been traced. For all spectra (the averaged and phase-resolved ones) in the energy range 5–55 keV we have searched for the cyclotron absorption line. The derived upper limit on the optical depth of the cyclotron line τ ~ 0.15 (3σ) points to the absence of this feature in the given energy range, which provides a constraint on the magnetic field of the neutron star: B <3 × 10¹¹ or >6.5 × 10¹² G. The latter constraint is consistent with the magnetic field estimate obtained by analyzing the pulsar’s power spectrum, B ? 3 × 10¹³ G. Based on our analysis of the phase-resolved spectra, we have determined the delay between the emission peaks and the equivalent width of the fluorescent iron line. This delay depends on the orbital phase and is apparently associated with the travel time of photons between the emitting regions in the vicinity of the neutron star and the region where the flux is reflected (presumably in the inflowing stream or at the place of interaction between the stream and the outer edge of the accretion disk).     

Boundary instability of the magnetized photosphere of a neutron star

The cyclotron resonance of electron scattering cross-section is shown to decrease the value of critical luminosity of neutron star below the classical Eddington limit corresponding to Thomson scattering. If the radiative flux prevails over critical one, instability of photospheric boundary in forms of ejection or MHD-pulsations might be evolved, giving rise to generation of hard gamma-photons and relativistic particles.     

Studies of accretion flows around rotating black holes – I. Particle dynamics in a pseudo-Kerr potential

We study the formation of the absorption features, called the cyclotron–annihilation lines, in the γ-spectra of the neutron stars (pulsars), owing to the fundamental quantum-electrodynamic effect of the one–photon pair creation in magnetized vacuum. As a result, we substantiate a new method for the determination of the neutron star magnetic fields B based on measuring the interval between the main annihilation and the first cyclotron–annihilation absorption lines. It is found that these lines may be easily resolved, and, consequently, the method is surely applicable if the following conditions are satisfied. (i) A γ-source has to be compact enough and located near a star, but not close to its magnetic poles. For instance, it may be a disc in the plane of a star magnetic equator with latitudinal angular width less than     and radial extent up to 25 per cent of the star radius. (ii) The source is to produce detectable γ-radiation at large angles ≳60° to the local magnetic field. Being situated in a closed field line region and having a broad radiation pattern, such a source is not what is usually considered in the context of the polar cap and outer gap models of the pulsar γ-emission dealing with open field lines only. (iii) Magnetic field strength must lie in a certain narrow interval with the centre at  ∼(3–4) × 10¹²  G. Its width depends on the star orientation and disc radial extend and in the most favourable case is about 20–30 per cent of its lower boundary. Finally, the influence of the star rotation on this method employment is considered and new possibilities arising from forthcoming polarization observations are briefly discussed.     

Thermal emission areas of heated neutron star polar caps

Observed hot spots on neutron stars are often associated with polar caps heated by the backflow of energetic electrons or positrons from accelerators on bundles of open magnetic field lines. Three effects are discussed that may be relevant to formation of hot spots and their areas. (1) The area of a polar cap is proportional to the ratio of the star’s surface dipole field to the local field at the polar cap. Because the field is coupled to the evolving spin of the superfluid core of the star, this ratio can depend on the stellar spin and its history. (2) The hot emission area may appear smaller to a distant observer when emitted X-rays propagate through electron-positron plasma created in the magnetosphere. The X-rays then change their energy spectrum because of cyclotron resonant scattering by pairs. (3) Hot spots may form on the star’s surface as a result of crust motions that are driven by the pull of core flux tubes pinned to the crust. Such motions twist the footprints of closed magnetic loops of the magnetosphere and induce an electric current in the loop, which will heat those footprints.     

On the possibility of disk-fed formation in supergiant high-mass X-ray binaries

We consider the existence of a neutron star magnetic field by the detected cyclotron lines. We collected data on nine sources of high-mass X-ray binaries with supergiant companions as a test case for our model, to demonstrate their distribution and evolution. The wind velocity, spin period and magnetic field strength are studied under different mass loss rates. In our model, correlations between mass-loss rate and wind velocity are found and can be tested in further observations. We examine the parameter space where wind accretion is allowed, avoiding the barrier of rotating magnetic fields, with robust data on the magnetic field of neutron stars. Our model shows that most sources(six of nine systems) can be fed by the wind with relatively slow velocity, and this result is consistent with previous predictions. In a few sources,our model cannot fit the standard wind accretion scenario. In these peculiar cases, other scenarios(disk formation, partial Roche lobe overflow) should be considered. This would provide information about the evolutionary tracks of various types of binaries, and thus exhibit a clear dichotomy behavior in wind-fed X-ray binary systems.     

X射线双星回旋吸收线研究进展

X射线双星中的回旋共振散射吸收特征(即回旋吸收线特征)是直接测量中子星磁场的工具。回旋吸收线表现为X射线能谱中多阶吸收特征。截至目前,已在30多颗源中探测到该现象,其能量范围为10-80 keV,对应的磁场强度范围为10^7-10^9 T。随着X射线探测技术的进步,回旋吸收线观测及理论研究也迅速发展,包括谐频和基频回旋吸收线能量之比、回旋吸收线形状的复杂性、回旋吸收线形态参数间的相关性、回旋吸收线能量与光度的关系、回旋吸收线的脉冲相位解析谱及回旋吸收线能量的长时标演化等。未来,人们将通过对回旋吸收线的研究,在探测高磁场中子星,以及在探究中子星磁场结构和吸积柱物理等方面取得更多成果。     

A new approximation for the high magnetic field Compton cross-section

A midly relativistic quantum-mechanical treatment of the Compton scattering cross-section in a strong magnetic field regime appropriate to observed electron temperature and field strengths in a neutron star magnetosphere is provided. The approximations used in the evaluation of the electron propagator are different from those employed previously and lead finally to simpler and more usable expressions for the cross-section.Various conceptual difficulties with the quantum cyclotron line mechanism are also discussed and the problem of the translational invariance of the Hamiltonian commonly used is addressed.